17.2. Natural and artificial selection
A subsection of Biology, 9700, through 17. Selection and evolution
Listing 10 of 69 questions
Spraying insecticide on the walls inside houses is the main method of controlling a species of Anopheles mosquito in rural India. A number of different insecticides have been used. Malathion was the main insecticide used for many years. In 2005 the newer insecticide, deltamethrin, was used instead and the use of malathion was discontinued. A laboratory study was carried out using mosquitoes collected from two sites in India. The percentage of mosquitoes killed by malathion and deltamethrin was estimated. The results of the study are shown in Table 4.1 Table 4.1 site year percentage of mosquitoes killed by malathion deltamethrin Jamnagar Bikaner With reference to Table 4.1, describe the difference in effectiveness of the two insecticides. The researchers concluded that at Jamnagar, the mosquitoes had evolved resistance to deltamethrin. Explain how the mosquitoes evolved resistance. Explain how the data in Table 4.1 show evidence that the use of malathion was discontinued after 2005. The resistance of mosquitoes to malathion was found to be due to a difference in the shape of one enzyme. Name the type of variation controlling malathion resistance in the mosquito population. Some students suggested that resistance to malathion could be due to a gene with two alleles. They proposed that the allele for resistance to malathion would be dominant to the allele for non-resistance. Using this assumption, the data in Table 4.1 can be used to calculate the frequency of resistant mosquitoes and the frequency of the allele for resistance in a mosquito population. Use the Hardy-Weinberg principle to calculate the frequency, p, of the allele for resistance in Jamnagar in 2005.
9700_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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Spraying insecticide on the walls inside houses is the main method of controlling a species of Anopheles mosquito in rural India. A number of different insecticides have been used. Malathion was the main insecticide used for many years. In 2005 the newer insecticide, deltamethrin, was used instead and the use of malathion was discontinued. A laboratory study was carried out using mosquitoes collected from two sites in India. The percentage of mosquitoes killed by malathion and deltamethrin was estimated. The results of the study are shown in Table 4.1 Table 4.1 site year percentage of mosquitoes killed by malathion deltamethrin Jamnagar Bikaner With reference to Table 4.1, describe the difference in effectiveness of the two insecticides. The researchers concluded that at Jamnagar, the mosquitoes had evolved resistance to deltamethrin. Explain how the mosquitoes evolved resistance. Explain how the data in Table 4.1 show evidence that the use of malathion was discontinued after 2005. The resistance of mosquitoes to malathion was found to be due to a difference in the shape of one enzyme. Name the type of variation controlling malathion resistance in the mosquito population. Some students suggested that resistance to malathion could be due to a gene with two alleles. They proposed that the allele for resistance to malathion would be dominant to the allele for non-resistance. Using this assumption, the data in Table 4.1 can be used to calculate the frequency of resistant mosquitoes and the frequency of the allele for resistance in a mosquito population. Use the Hardy-Weinberg principle to calculate the frequency, p, of the allele for resistance in Jamnagar in 2005.
9700_w16_qp_42
THEORY
2016
Paper 4, Variant 2
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Meiosis is one process that contributes to genetic variation. State precisely the stage of meiosis where single chromosomes line up on the equator. Outline the events taking place during anaphase I of meiosis. Describe how crossing over during meiosis leads to genetic variation. Mutation also causes genetic variation. Some populations of water hemp, Amaranthus tuberculatus, have evolved herbicide resistance as a result of a mutation. This is a problem for farmers as water hemp grows in crop fields, lowering productivity. Two populations of water hemp were tested for resistance to the herbicide mesotrione. One was a population known to be resistant and the other was a test population, whose resistance was unknown. • Leaves were removed and immersed in a radioactively labelled solution of mesotrione. • The leaves absorbed some mesotrione and became radioactive. • Resistant leaves are able to degrade mesotrione by metabolism. • The time for 50% of absorbed mesotrione to degrade was calculated by measuring the radioactivity of the leaves. The results are shown in Table 3.1. Table 3.1 population of water hemp mean time for 50% of absorbed mesotrione to degrade / hours standard deviation test 27.5 4.75 control 10.1 2.34 Explain how the results in Table 3.1 show that the two populations differ in their resistance to mesotrione. Explain why this example of genetic variation is important for natural selection in water hemp populations. Farmers can send in a sample of leaves of water hemp from their fields to a laboratory to be tested for resistance to mesotrione or other herbicides. Suggest the benefit of this to a farmer. The null hypothesis states there is no significant difference between the mean times for 50% of absorbed mesotrione to degrade in the two populations. A t‑test can be carried out to compare these two means. The critical value for t at the p = 0.05 significance level is 2.23. Use the formula in to calculate the value of t. Show your working. t n s n s x x = + - e o Key x = mean s = standard deviation n1 = 6 (number of readings for test population) n2 = 6 (number of readings for control population) t = Use your calculated value of t to explain whether the null hypothesis should be accepted or rejected. accept or reject explanation
9700_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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Researchers have found evidence of natural selection in humans. • Originally, in human populations it was only babies and children that needed to digest the milk sugar, lactose. The gene coding for the enzyme lactase (LCT gene) was switched off before adulthood. • Today, in many populations, some adult individuals have lactose intolerance, which means they cannot digest lactose. Lactose intolerance leads to side-effects such as abdominal pain after eating food containing lactose. • A mutation has been identified that keeps the LCT gene switched on. An adult who has this mutation is able to digest lactose. This is called lactose persistence. • Lactose persistence increased in populations in Europe several thousand years ago. • The increase in lactose persistence in Europe coincided with an increase in farming of cows for milk. Natural selection has caused this increase in lactose persistence. State the type of selection that has caused this increase. Explain why there was selection for lactose persistence in humans several thousand years ago. Lactose intolerance and lactose persistence were investigated in a test population in Europe. The mutation which causes lactose persistence is in a regulatory gene (T/t). • People with lactose intolerance have the genotype tt. • People with lactose persistence have the genotypes TT and Tt. • 166 people were tested for their genotype. • 58 people were found to have lactose intolerance. The Hardy–Weinberg principle can be used to calculate allele, genotype and phenotype frequencies in populations. The Hardy–Weinberg equations are shown in : p + q = 1 p2 + 2pq + q2 = 1 Calculate the frequency of allele T. Show your working. frequency of allele T = When the calculated phenotype frequencies were compared to those in the general population in Europe, it was found that the percentage of people with lactose intolerance in this test population was much higher than in the general population. Suggest two reasons why the percentage of people with lactose intolerance was much higher in the test population than in the general population. In eukaryotes, gene expression is controlled by transcription factors, coded for by regulatory genes. Outline ways in which transcription factors carry out their role. It is estimated that 2% of human DNA consists of genes coding for proteins (structural genes). Of the remaining 98%, some of the DNA consists of regulatory genes and control sequences that together control gene expression. State one type of control sequence found in human DNA. A study of human evolution identified the location of mutations that result in a change in human phenotype. The study found most examples of mutations had occurred in regulatory genes, not structural genes. Suggest and explain why most changes in human phenotype are due to mutations in regulatory genes.
9700_w18_qp_42
THEORY
2018
Paper 4, Variant 2
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Meiosis is one process that contributes to genetic variation. State precisely the stage of meiosis where single chromosomes line up on the equator. Outline the events taking place during anaphase I of meiosis. Describe how crossing over during meiosis leads to genetic variation. Mutation also causes genetic variation. Some populations of water hemp, Amaranthus tuberculatus, have evolved herbicide resistance as a result of a mutation. This is a problem for farmers as water hemp grows in crop fields, lowering productivity. Two populations of water hemp were tested for resistance to the herbicide mesotrione. One was a population known to be resistant and the other was a test population, whose resistance was unknown. • Leaves were removed and immersed in a radioactively labelled solution of mesotrione. • The leaves absorbed some mesotrione and became radioactive. • Resistant leaves are able to degrade mesotrione by metabolism. • The time for 50% of absorbed mesotrione to degrade was calculated by measuring the radioactivity of the leaves. The results are shown in Table 3.1. Table 3.1 population of water hemp mean time for 50% of absorbed mesotrione to degrade / hours standard deviation test 27.5 4.75 control 10.1 2.34 Explain how the results in Table 3.1 show that the two populations differ in their resistance to mesotrione. Explain why this example of genetic variation is important for natural selection in water hemp populations. Farmers can send in a sample of leaves of water hemp from their fields to a laboratory to be tested for resistance to mesotrione or other herbicides. Suggest the benefit of this to a farmer. The null hypothesis states there is no significant difference between the mean times for 50% of absorbed mesotrione to degrade in the two populations. A t‑test can be carried out to compare these two means. The critical value for t at the p = 0.05 significance level is 2.23. Use the formula in to calculate the value of t. Show your working. t n s n s x x = + - e o Key x = mean s = standard deviation n1 = 6 (number of readings for test population) n2 = 6 (number of readings for control population) t = Use your calculated value of t to explain whether the null hypothesis should be accepted or rejected. accept or reject explanation
9700_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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The Hardy-Weinberg principle may be used to calculate allele and genotype frequencies for a gene, within a population. The Hardy-Weinberg principle uses these equations: p2 + 2pq + q2 = 1 p + q = 1 Within a population of butterflies, the allele for brown wing colour, B, is dominant to the allele for white wing colour, b. 40% of the butterflies in this population are white. Use the Hardy-Weinberg principle to calculate the percentage of butterflies in the population that are heterozygous for the gene controlling wing colour. Show your working. answer % Describe four situations where the Hardy-Weinberg principle does not apply.
9700_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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Questions Discovered
69